The present invention relates to a planar filter with ferroelectric and/or antiferroelectric elements.
Such a planar filter with ferroelectric and/or antiferroelectric elements is disclosed for example in the patent document WO 94/28592. In this filter a ferroelectric or antiferroelectric layer is mounted on a dielectric substrate. The microstructured high temperature super-conductive layer is arranged on the layer substrate and in particular on its upper side, while an unstructured high temperature super conductive layer is also arranged on the lower side. Together they form a band pass filter in the microstrip conductor form. A planar electrode is located several millimeters above the upper superconductive structure. By applying a voltage between the upper high temperature superconductive layer and the planar electrode, the effective dielectric constant of the intermediate space between the structure superconductive layer and the unstructured super-conductive layer can be changed since the dielectric constant of the ferroelectric or the antiferroelectric substantially varies in dependence on the applied voltage. Thereby the filter characteristic also changes, in particular its transmission frequency.